This invention relates to a pattern transfer apparatus and, more particularly, to a pattern transfer apparatus to be used in the manufacture of semiconductor circuit devices.
Referring to FIG. 1, a known type pattern transfer apparatus will be first described. The pattern transfer apparatus shown in FIG. 1 is of reduction projection and step-and-repeat type, called a stepper. The transfer apparatus includes a projection lens 1 for forming, on an image plane 4, an image of a reticle 2 disposed at a predetermined location. The projection lens 1 is provided with non-contact type sensors 6 - 6 which are fixedly secured to the barrel of the projection lens 1 in order to detect the position of the surface of a wafer 3 held by a wafer carriage 5 when the wafer 3 is moved to the pattern transfer station below the projection lens 1. The non-contact type sensors 6 - 6 may comprise air-sensor nozzles or capacitance sensors. Since high precision measurement is effected by these sensors 6 - 6, a gap G between the image plane 4 and these sensors must be maintained very small.
The pattern transfer apparatus further includes an X-Y stage 7 which is movable in parallel to the image plane 4 of the projection lens 1 and has a Z-axis driving mechanism 8 for moving upwardly and downwardly, as viewed in FIG. 1, the wafer carriage 5 and the wafer 3 held by the wafer carriage 5. The wafer 3 is first moved to a position below the projection lens 1 by the X-Y movement of the stage 7 and then is gradually moved upwardly by the Z-axis driving mechanism 8 in accordance with the outputs of the noncontact type sensors 6 - 6 monitoring the distance to the wafer 3 surface, so that the wafer 3 surface is coincident with the image plane 4. Disposed adjacent to the projection lens 1 is an alignment detecting system 10 for detecting a particular pattern or patterns formed on the wafer 3. The alignment detecting system 10 is fixedly secured in a predetermined positional relation with the projection lens 1. The alignment detecting optical system 10 acts as a major alignment-detecting system in the case where the transfer apparatus is of the off-axis alignment type and acts as an auxiliary alignment-detect in the case where the transfer apparatus is of the TTL (Through The Lens) alignment type. The object plane 11 of the alignment detecting system 10 may be on the same plane of the image plane 4 of the projection lens 1, or may be on a plane slightly lower than the image plane 4.
The extent of movement of the X-Y stage 7 is determined such that every portion of the surface of a wafer carried by the carriage 5 can be positioned within the picture field of the projection lens 1, that at least a particular area of the wafer can be observed by the alignment detecting system 10, and that the wafer can be moved to a predetermined wafer loading/unloading position 12. On the other hand, the extent of movement of the Z-axis driving mechanism 8 is determined such that the position of the surface of a wafer having a presumably maxiumum thickness and carried by the carriage 5 when it is in its lowermost position comes lower than the object plane 11 of the alignment detecting system 10 and that the surface of a wafer having a presumably minimum thickness and carried by the carriage 5 can be moved to a position higher than the image plane 4 of the projection lens 1.
In operation of the transfer apparatus shown in FIG. 1, a wafer onto which the pattern of the reticle 2 has been transferred is replaced by a new, not yet exposed, wafer 3 at the wafer loading/unloading position 12. The newly placed wafer 3 is fixedly held by the carriage 5. The loading/unloading of the wafers is carried out with the Z-axis driving mechanism 8 being maintained at its reference position which is usually the lowermost position. Subsequently, the wafer 3 is moved to the pattern transfer station, i.e., the position which is below, as viewed in FIG. 1, the projection lens 1 by means of the X-Y stage 7. Since in the FIG. 1 arrangement the thickness of the wafer 3 carried by the carriage 5 is not at all detected, Z-axis movement (upward movement in FIG. 1) should not be initiated during the X-Y movement of the stage 7. If, to the contrary, the wafer 3 surface is moved upwardly beyond the image plane 4 of the projection lens 1 during the X-Y movement of the stage 7, the wafer 3 surface will collide against the barrel of the projection lens 1 because the gap G is very small as described in the foregoing. In view of this, the upward movement of the wafer 3 is initiated after the stage 7 and therefore the wafer 3 reach the pattern transfer position under the projection lens 1. The Z-axis movement is carried out while measuring the distance between the wafer surface and the end face of the projection lens 1 by means of the non-contact sensors 6 - 6. When the wafer 3 surface is coincident with the image plane 4, the Z-axis movement is terminated. If the wafer 3 is virgin, i.e. if the reticle 2 is the first reticle, it is not necessary to align the reticle 2 and the wafer 3 with each other. Therefore, an exposure step is initiated immediately after completion of the Z-axis movement. If the reticle 2 is not the first reticle, i.e. if any pattern has been formed on the wafer 3 during the preceding pattern transfer operation, the alignment between the reticle 2 and the wafer 3 is necessary. In such case, the X-Y stage 7 is moved to move the wafer 3 surface from the position of the image plane 4 to the position of the object plane 11 in order that the alignment mark formed on the wafer 3 is positioned within the viewing field of the alignment detecting system 10 to achieve alignment of the particular pattern already formed on the wafer 3 surface. As described in the foregoing, the object plane 11 of the alignment detecting system 10 is on the same plane as the image plane 4 of the projection lens 1 or on the plane lower than the image plane 4. In the latter case, Z-axis movement to the object plane 11 is effected simultaneously with the movement of the X-Y stage 7. The difference or deviation A between the imaging plane 4 of the projection lens 1 and the object plane 11 of the alignment detecting system 10 is a fixed value for a particular transfer apparatus. Therefore, by designating Z-axis movement of an amount A, the wafer 3 surface becomes coincident with the object plane 11.
Such operation is illustrated in FIG. 2 with respect to the locus of the displacement of a particular point on the wafer 3 surface. The locus is shown by a broken line. Apparently, the movement to the object plane 11 of the detecting system 10 includes useless motion which requires useless time. This inefficiency leads to a decreased throughput.
It will be possible to provide a separate focus detecting system adjacent to the alignment detecting system 10. If, in such case, the Z-axis movement is made simultaneously with the movement of the X-Y stage 7 so that the wafer 3 surface is directly moved to the object surface 11, the locus of displacement of the particular point on the wafer 3 surface will be such as shown by a dash-and-dot line or a dash-and-dots line. Whether the displacement follows the dash-and-dot line or the dash-and-dots line depends on the thickness of the wafer 3. Particularly, in the former case, however, the wafer 3 surface will possibly collide against the projection lens 1 because the alignment detecting system 10 is disposed close to the projection lens 1. Even if there is no limitation by the projection lens 1 due to the presence of the gap G, discrimination of the direction of movement to be made and measurement of the distance are still necessary. As the result, it is still necessary to gradually move the wafer 3 surface toward the object surface 11. This requires a prolonged time for the operation.